To properly carry out the mission thereof, the fish is provided with suitable hydrodynamic features. For example, the fish is configured to develop significant vertical forces that allow it to dive to a desired depth despite the towing speed that tends to make it rise to the surface.
The simplest way to develop these vertical forces is to increase the weight of the fish beyond buoyancy. The weight has the advantage of being constant irrespective of the speed of the ship that pulls the fish. Another means consists in arranging, on the fish, a set of hydrodynamic fins, the lift of which is directed downwards. Given the density of water, these fins are quite short and wide and are housed easily on a towed body. The downward hydrodynamic lift force increases with the square of the speed of the fish with respect to the water. As a result, the greater the speed, the more the fish is kept submerged.
Towed sonars are mainly used in the military field. It can be that the ship operating with the sonar thereof submerged at depth suddenly has to escape, for example, if pursued by a torpedo. In this case, given the urgency, it is not possible to bring the towed body back on board. It must be possible for the escape speed of the ship to be supported by both the towed body and the fish towing cable. Indeed, the cable is subjected to a hydrodynamic drag which increases with speed.
The cable must be sized to withstand a maximum tension generated by the ship for a given escape speed. Other elements, such as the towing machine and the attachments thereof on the deck of the ship, must also be sized to resist this maximum tension. A high escape speed requires all of the elements involved in the towing operation to be oversized. Oversizing the cable leads to increasing the dimensions thereof and to further increasing the hydrodynamic drag thereof and, consequently, the pulling effort on the cable.
During the escape of the ship, in order to limit the tension of the cable, one maneuver consists in bringing back on board as much cable as possible. To allow such a maneuver, the towing winch must also be oversized.
For a given piece of equipment, there are two solutions. The first is the limitation of the escape speed of the ship and the second, in an extreme case, is to cut the tow line in order to avoid untimely breaking. This, of course, involves the loss of the towed body.
The aim of the invention is to overcome all or some of the problems mentioned above by proposing to limit the hydrodynamic efforts on the fish, if necessary.
To this end, the object of the invention is a fish intended to be submerged and towed by a cable, the fish comprising a supporting structure configured to move in water in a horizontal main direction and at least one appendage configured to generate, for the fish, a hydrodynamic lift directed downwards when the fish moves in the water under the towing effect, characterized in that the appendage is orientable between a first position generating a maximum value of the hydrodynamic lift of the fish and a second position generating a reduced value of the hydrodynamic lift of the fish, in that the fish comprises a lock which, in a locked configuration, keeps the appendage orientated in the first position and which, in an unlocked configuration, frees the orientation of the appendage, and in that the appendage is configured to move from the first position to the second position under the effect of a hydrodynamic lift specific to the appendage.
By reducing the hydrodynamic lift of the fin, the fish tends to rise towards the surface of the water. This advantage can be useful in cases other than the escape of the ship. When there is an unforeseen shoal, the towed body can abut against this shoal leading to a risk of the tow line breaking and the towed body being lost. Slowing down the ship would only aggravate the situation by making the towed body dive. By using the means to reduce the hydrodynamic lift of the fin, the fish tends to rise and is, thus, in a better position to avoid the shoal.